Integrated circuits are incorporated in many types of electrical equipment. Such integrated circuits are generally vulnerable to damage from high voltage transients. In some equipment, high voltage transients may have positive and/or negative peak levels of 100 volts or more and may have a duration of several microseconds. High voltage electrostatic discharge (ESD) transients can also result from a user becoming electrostatically charged, for example, by friction or by induction and touching equipment controls.
Protection devices applicable to the protection of integrated circuits from damage that would otherwise result from a high voltage transient are known in the art. Such devices are described in, for example, Avery, U.S. Pat. No. 4,414,711; Avery, U.S. Pat. No. 4,405,933; Kokado et al., U.S. Pat. No. 4,631,567; and Rountree et al., U.S. Pat. No. 4,692,781.
In providing its protective function, it is also desirable that a protection device be able to handle transients associated with relatively large energy without itself being destroyed or having its protective capability significantly impaired. Furthermore, it is desirable that the protective device provide an indication of having been subjected to a large voltage transient.